Most of LED substrates are now made from sapphire single crystal boules through cutting, grinding, oriental sectioning and polishing single crystal sapphire boules. Recently, sapphire single crystal has also found wide applications in smart phones, tablet computers, and other consumer electronic devices. High purity aluminum oxide is the most important precursor for sapphire singe crystal growth.
The sapphire single crystal is produced from aluminum oxide melts at high temperature. Popular methods for sapphire growth include Czochralski method, Edge-defined Film-fed Growth, Vertical Gradient Freeze method, Kyropoulos method, and the like.
Defects inside a sapphire crystal often include dislocations, bubbles, inclusions and cracks. These defects can cause absorption, scattering, reflection or diffraction of photons, phonons or other electromagnetic energy, and can therefore reduce the overall quality of sapphire substrates. Consequently, these defects can significantly impact the performance of LEDs made from the substrates with these types of defects. Furthermore, these defects can also result in discoloring of the sapphire crystal. The formation of defects in the sapphire crystal growth is believed to be due to high level of impurities, especially high metal contents in the aluminum oxide raw material. Therefore, it is often recommended to control overall metal contents, as well as silicon impurity, within less than 0.005% in the aluminum oxide raw material for high quality sapphire crystal manufacturing.
In addition, if the aluminum oxide powder is used as a raw material for sapphire crystal growth, the issues also include dusting during packing process and low batch quantity of the raw material. Because the sapphire growth is a batch process, low packing quantity can reduce the manufacturing efficiency, and therefore impact the overall sapphire growth capability and cost control. For the aluminum oxide raw material currently used in the sapphire growth process, overcoming the two major issues, i.e., high metal and silicon contents and low packing quantity, is highly desirable.